A retardation film, comprising a transparent optical film in which the birefringence thereof is controlled, has been utilized to increase the viewing angle of a liquid crystal display device. Most of the above retardation films have been produced employing materials exhibiting the highest refractive index in the orientation direction of molecules, the so-called positive optical anisotropy. Other than the retardation film, in a triacetate cellulose film (a TAC film), which is used as a polarizing plate protective film, the triacetate cellulose therein also exhibits a small but positive optical anisotropy. However, in case that phase difference is applied to the polarizing plate protective film itself, since materials exhibiting positive optical anisotropy, such as cellulose ester, cycloolefin, or polycarbonate, are stretched for the production of the polarizing plate protective film, the in-plane refractive index of the film is maximized. Therefore it was difficult to produce a retardation film having a low in-plane refractive index, a so-called positive C-plate. To produce the positive C-plate, there have been disclosed methods, one being to negate birefringence of the film by incorporating a material exhibiting negative optical anisotropy, and the other being to apply negative optical anisotropy to the entire film by increasing the additive amount of the above material these materials, polystyrene has been known as a material exhibiting significant negative birefringence (for example, refer to Patent Documents 1 and 2). However, since polystyrene is less likely to be compatible with materials such as a cellulose ester which were used for producing the polarizing plate protective film, the polystyrene exhibited a problem in that a large amount thereof could not be incorporated. The cellulose ester also exhibited a problem in that dimensions of the film tended to change due to water being absorbed and/or repelled by the film.
Patent Document 1: Japanese Patent Application Publication (hereinafter also referred to as JP-A) No. 2002-156624
Patent Document 2: JP-A No. 2005-105140